Forward looking infrared (FLIR) sensors are infrared imaging devices that utilize a scanning mirror for scanning the infrared energy from an external scene onto an infrared detector. Typically, the scanning mirror is a rotatable mirror disposed in a housing, and the detector is a solid state device such as a solid state detector array. The output of the detector is an electronic signal suitable for display on a CRT or other output device.
A problem which is often encountered in FLIR sensors is that each element or pixel of the detector has a different sensitivity to infrared energy. The same amount of infrared energy may cause different elements of the detector to produce different output levels. Accordingly, there is a need to provide a reference for the detector in order to normalize the outputs of the individual elements of the detector.
One prior art technique for referencing a FLIR sensor included thermal references which were disposed at a roll-stabilized focal plane within the afocal optics. The thermal references were then imaged directly onto the detector. The disadvantages of this prior art technique are that direct illumination requires extremely tight source temperature uniformity, a requirement for a stabilized focal plane, and overscan of the field of view.
Another prior art technique for referencing FLIR sensors is described in U.S. Pat. No. 4,948,964, entitled "Artificial Target for Automatic Gain Normalization". The device described in U.S. Pat. No. 4,948,964 includes a servo controlled chopper mirror that views a single thermal reference. The chopper mirror has two different reflectivites, and therefore is capable of providing two different effective reference temperatures. The relationship between the two reference temperatures is fixed, because the relationship is dictated by the structure of the chopper mirror. The chopper mirror also has a relatively large moment of inertia due to the increased size necessary to accommodate the two different reflective surfaces. Since the plane of the chopper mirror is substantially perpendicular to the axis of rotation of the servomotor, the mirror assembly is a relatively bulky device which cannot be easily retrofitted into an existing sensor.
Accordingly, there is a need for a substantially more compact and less bulky thermal reference system which is suitable for being retrofit into existing sensors. Such an improved thermal referencing system must easily fit within the optical path of an existing sensor. Moreover, the relationship between reference temperatures should be easily modified without redesigning the structure of the mirror.